Communication systems, including cellular mobile telephone systems and networks are making increasing use of digital technology. Cellular mobile telephone networks require a wireless radio communication link to be established between a mobile terminal and a basestation. Second generation cellular mobile telephone systems exchange digital signals over the wireless radio communications link.
Digital systems can be exploited to provide greater spectral efficiency of a radio communications link than is offered by an analogue system and digital processing can often minimise the effects of interference.
In communication systems that rely on wireless links, such as mobile communications systems, the quality of these links can vary considerably. A number of factors influence the quality of the link and the system must be tolerant of any such variation. In systems employing an analogue link, a reduction in link quality may merely result in a noisy but tolerable link being established. However, in systems employing a digital link, it is important that the information sent over the link can be faithfully recovered at the receiving end, even when the link quality is poor. The effect of incorrectly received information depends on the application. For example, in the case of a digital cellular mobile radio telephone system employing a wireless radio link, during a telephone conversation, incorrect reception and loss of information over the link may just result in a temporary muting of sound. However, with the advent of mobile computing, mobile telephone cellular networks are increasingly used for the communication of data and in this situation any loss of data is unacceptable.
Various techniques are known for assisting with correct communication of digital information over a range of media and some of these techniques fall into the category of error detection and correction. One technique is forward error correction (FEC) which involves encoding information prior to transmission in such a way that any errors occurring during communication may be identified and corrected on reception. Another technique is to employ an automatic repeat request (ARQ) error control scheme which involves the retransmission of information that is deemed to have been erroneously received or not received at all. There are various derivatives of the basic ARQ scheme and these are employed depending on the feasibility of providing buffer space at the transmit/receive ends of the link and the requirement to utilise the link efficiently. Indeed some ARQ schemes do not merely retransmit the same information. In the case of these schemes retransmission involves the (re)transmission of only a portion of the information, transmission of appropriate FEC information or any combination thereof. Various ARQ schemes are well known to the person skilled in the art, as is the fact that retransmission may be initiated in the absence of an acknowledgement that information has been received correctly or even incorrectly. This is in contrast to the situation where an explicit request for retransmission is sent to the transmitter. When FEC and ARQ techniques are combined they can provide a powerful error detection and correction mechanism and in certain implementations ARQ operation only becomes active if FEC fails to recover information. However both techniques are most effective when the quality of the link is more predictable and consistent as may be provided, for example, by a co-axial cable although wired twisted pair links are susceptible to noise. In contrast, with wireless radio communication links, such as those employed between mobile cellular telephones and basestations, the link quality is constantly changing due to the movement should the terminal be mobile, obstructions caused by buildings, the geography of the area, weather conditions and the distance of the wireless link. In particularly poor conditions ARQ techniques will result in multiple retransmissions occurring which may cause a delay in data communication and an overall increase in system power consumption. Where components of the system, for example mobile terminals, rely on battery power this is particularly undesirable.
Where the radio link quality deteriorates because of large signal fluctuations, for example at the onset of a deep fade, various prior art remedies have been proposed.
U.S. Pat. No. 5,105,423 describes a method of shifting down a data transmission rate in response to a request from a receiver for retransmission of data frames having errors in them. The data transmission rate is shifted down from a first data rate to a second data rate which is used for retransmission. The shift down rate is calculated based on the possibility of errors re-occurring during retransmission, which is in turn calculated on the basis of the percentage of frames that need to be re-transmitted. The reduction in transmission rate is undesirable in certain applications especially those where rapid data transmission is a priority.
U.S. Pat. No. 5,128,965 describes a radio link system where overall transmission power is increased should the receiver indicate that the number of errors in the received information exceeds a particular threshold. A system described in U.S. Pat. No. 5,713,074 employs power controller apparatus to increase transmit power when an error is detected in a transmitted signal, and to reduce the transmit power by a smaller amount when no error is detected, thus maintaining a constant bit error rate. Error detection is facilitated by returning the received signal to the transmitter and comparing the received signal to that transmitted.
The standard TIA/EIA-95-B published on 3rd Feb. 1999 by the Telecommunications Industry Association is developed from interim standard TIA/EIA/IS-95 which describes the process where a mobile terminal signals a fixed system over the air interface for the purpose of gaining access. If the mobile terminal does not receive a reply from the fixed system, the mobile terminal repeats the signalling process but with an increased transmission power. This signalling process is concerned only with the task of obtaining access and not with the transfer of traffic.